Various improvements have been made in the properties of absorbent articles, such as sanitary napkins, paper diapers, incontinence pads and panty liners, including the “fluid absorption rate” relating to the speed of absorption of fluids, the “fluid diffusibility” relating to the ability of an absorbed fluid to diffuse through the entire absorbent body, the “absorbed fluid retention volume” relating to the amount of absorbed fluid that can be retained, and the “fluid retention” relating to the ability to retain absorbed fluids, for the purpose of preventing fluid leakage and reducing sticking of the absorbent article to the body, as well as the “low twisting” property.
Conventionally, absorbent sheets produced using natural pulp fiber as starting materials, and pulp fiber laminated sheets, such as fluff pulp sheets and dry pulp nonwoven fabrics, have been used as absorbent articles and especially as the materials of absorbent bodies, and it has been attempted to control the aforementioned properties by modifying the density of the absorbent body or employing super-absorbent polymer particles.
The absorbent sheet is generally prepared by wet paper forming and sheeting, and has a dense fiber layer structure. Such absorbent sheets are therefore considered to have strong capillary movement and excellent diffusibility for absorbed fluids. In actuality, however, such absorbent sheets do not exhibit satisfactory “fluid diffusibility” because they essentially lack voids between fibers, while they also lack a satisfactory “absorbed fluid retention volume” and “fluid absorption rate” because of the low degree of fluid retention.
Moreover, although the absorbent sheets have very high dry strength due to hydrogen bonding of cellulose, since the pulp fibers are densely layered, the absorbent sheets lose the hydrogen bonding between cellulose fibers when in a wet state, such that the wet strength becomes essentially zero, resulting in twisting and tearing during use. While the wet strength can be increased by using wet strengtheners, the low thicknesses tend to result in deformation, or twisting tends to occur due to the lack of repulsion elasticity.
Fluff pulp has a loose fiber layer structure with very weak bonding strength between fibers and numerous voids between fibers. Therefore, fluff pulp has a very high “fluid absorption rate” and a large “absorbed fluid retention volume”, but conversely to the aforementioned absorbent sheets, its voids between fibers are too large, resulting in inferior “fluid diffusibility”.
Such fluff pulp has elasticity in a dry state as well, but it also lacks any appreciable bonding strength between fibers, and therefore exhibits low strength and is prone to deformation, tearing and twisting during use.
Moreover, fluff pulp decreases in bulk when wetted by absorbed fluids, and application of body pressure tends to cause further reduction in bulk and to result in a very low absorption space. Therefore, since body fluids that have been absorbed and retained flow back to the body side and cause sticking and leaking, such pulp is inferior in terms of “fluid retention”.
Dry pulp nonwoven fabrics are nonwoven fabrics produced by layering pulp fibers into a sheet in air and bonding the fibers together with a binder, and they have high wet strength, as well as a loose fiber built-up structure with a relatively large number of voids between fibers. Thus, the “absorbed fluid retention volume” is high upon wetting; however, the pulp fibers swell and soften and readily undergo deformation upon application of body pressure, and absorbed and retained body fluid returning back to the body side causes sticking and leaking, such that twisting or deformation tends to occur during use, similar to fluff pulp, and therefore the “fluid retention” is poor.
Fiber starting materials other than natural pulp fiber include bulky crosslinked cellulose fiber, comprising cellulose fiber with a fiber roughness of 0.3 mg/m or greater that has been crosslinked within and/or between molecules with a crosslinking agent, as disclosed in PTL 1. The crosslinked cellulose fiber disclosed in PTL 1 has low swelling when wet and is resistant to twisting and flattening, and is therefore useful as an absorbent body for absorbent articles.
Because the crosslinked cellulose fiber disclosed in PTL 1 is fibrous similar to natural pulp fibers, it must be worked into a form, such as an absorbent sheet, fluff pulp or dry pulp nonwoven fabric, similar to natural pulp fiber, when it is to be used as an absorbent body for an absorbent article.